Project Summary/Abstract While adverse health consequences of smoking make it the primary cause of preventable mortality in the world, current therapies for smoking cessation are minimally effective highlighting the need for a better understanding of the pathophysiology underlying nicotine addiction. Recent work has identified an understudied circuit, the habenulo-interpeduncular nucleus (Hb-IPN) axis, as a critical circuit in nicotine withdrawal symptoms. In particular, during nicotine withdrawal in mouse models, neurons within the IPN exhibit increased activity triggering both somatic (physical) withdrawal symptoms, as well as affective symptoms including increased anxiety. Although little is known regarding the neurotransmitter input and receptor signaling within the IPN that modulates neuronal activity, previous work and preliminary data indicate serotonergic neurons originating in the median raphe nucleus innervate the IPN. Thus, the goal of this R21 is to identify a role for IPN serotonin and serotonergic receptor signaling in nicotine withdrawal behaviors. Aim 1 will test the hypothesis that median raphe serotonergic neurons project to, and modulate the firing of IPN neurons via serotonin receptor signaling. This hypothesis will be tested using a combination of optogenetic, biophysical, molecular, and pharmacological approaches in acute midbrain slices. Aim 2 will test the hypothesis that activation of serotonergic inputs into the IPN in vivo will modulate somatic and/or affective nicotine withdrawal symptoms in a mouse model of nicotine dependence. This hypothesis will be tested using in vivo optogenetic approaches in combination with behavior and brain region specific drug infusions. It is anticipated that characterizing the role of serotonin in the IPN during nicotine withdrawal will lead to identification of novel serotonin receptor targets for smoking cessation.